Along with the reduction in weight and improvement in performance of automobiles, springs are being made higher in strength. High strength steel having a tensile strength exceeding 1500 MPa after heat treatment is being used for springs. In recent years, steel wire having a tensile strength exceeding 2100 MPa is also being sought. This is so as to secure a hardness of material where even with some softening due to stress-relief annealing, nitridation, and other heating at the time of spring production, there is no problem for the spring.
Further, it is known that with nitridation or shot peening, the surface hardness rises and the durability during spring fatigue is remarkably improved, but the spring setting characteristic is not determined by the surface hardness. The internal strength or hardness of the spring material also has a great effect. Therefore, it is important to design compositions able to maintain the internal hardness extremely high.
As a technique for this, there is an invention adding V, Nb, Mo, or another element to form fine carbides dissolving by quenching and precipitating by tempering and thereby limiting the movement of dislocations and improving the anti-setting property (for example, see Japanese Patent Publication (A) No. 57-32353).
On the other hand, among the methods for production of steel coil springs, there are “hot coiling” comprising heating the steel to the austenite region for coiling, then quenching and tempering it and “cold coiling” comprising quenching and tempering the steel in advance and cold coiling the resultant high strength steel wire. With cold coiling, it is possible to use oil tempering, high frequency treatment, etc. enabling rapid heating and rapid cooling at the time of production of the steel wire, so it is possible to reduce the grain size of the prior austenite of the spring material. As a result, it is possible to produce a spring superior in fracture characteristics. Further, it is possible to simplify the heating furnace and other facilities on the spring production line, so there is the advantage to the spring manufacturers as well that this leads to a reduction in the capital costs. Recently, springs are being cold worked. In suspension springs, compared with valve springs, thicker steel wires are used, but cold coiling is introduced due to its advantages.
However, if the spring-use steel wire for cold coiling increases in strength, it will break at the time of cold coiling and will be unable to be formed into a spring shape in many cases. Up to now, both strength and workability could not be achieved, so the wire had to be coiled by industrially disadvantageous methods of hot coiling or coiling, then quenching and tempering to obtain both strength and workability.
Further, when cold coiling and nitriding high strength heat treated steel wire to secure its strength, it has been believed effective to add large amounts of V, Nb, and other so-called alloy elements causing precipitation of fine carbides in the steel. However, if actually adding large amounts, they will not dissolve by the heating at the time of quenching, but will grow coarser and form so-called undissolved carbides which will become factors behind fracture at the time of cold coiling. Therefore, technology focusing on the undissolved carbides has also been seen.
There is an invention aiming at improvement of performance by controlling not only the alloy elements, but also the carbides such as the cementite present in large amounts in the steel (for example, see Japanese Patent Publication (A) No. 2002-180198).
These patents prescribe the spherical carbides in detail and try to achieve both workability and higher spring strength, but even if suppressing the carbides of such relatively clear spherical carbides (alloy-based and cementite-based), there are limits to further increase of the strength and improvement of the spring performance. That is, these provisions by nature strongly suppress “defects” and suppress deterioration of the workability. There were also limits to the direct strengthening of the spring performance.